This invention relates to pen plotters and control apparatus therefor and, more particularly, to a dual-function single-sensor system for use in a pen plotter wherein plotting media is moved over a platen by a pinch roller and drive drum assembly controlled by plotter control logic in response to commands from a user through a control console and the plotter control logic also turns a motor on when needed for plotting and off when not needed for plotting, comprising, a slot in the platen adjacent the pinch roller and drive drum; a source of a light beam disposed on one side of the slot and directing the light beam through the slot; a photodetector disposed on an opposite side of the slot to receive the light beam and provide an electrical signal at an output thereof, the output being connected to the plotter control logic to be sensed thereby whereby the plotter control logic can determine the presence of plotting media breaking the light beam; and, supplemental logic means included within the plotter control logic for turning the motor on when the presence of plotting media breaking the light beam is detected and for turning the motor off when the presence of plotting media breaking the light beam is not detected and for allowing a loading and plotting sequence of the plotter control logic to begin only after a start command is input by a user through the control console and the presence of plotting media breaking the light beam is detected.
In a typical pen plotter, apparatus is included as depicted in simplified form in FIG. 1. There is plotter control logic 10 connected to control the operation of a plotter mechanism 12 and start servo fan motors 14 partially as a function of inputs from various sensors 16. The plotter operates by a sheet of plotting media being inserted between a drive drum and pinch rollers. The pinch rollers are lowered to hold the media against the drive drum. Thus, as the drive drum rotates forward and backward, the media is correspondingly moved forward and backward under a beam carrying a plotting head which moves transversely across the media. The movement of the media in combination with the movement of the plotting head creates movement of a plotting pen carried by the plotting head in both directions of an X-Y co-ordinate system of the plotter. This, of course, comprises what is represented in FIG. 1 by the plotter mechanism represented by block 12 and is all well known and standard in a pen plotter of common design.
Typically, the plotter control logic 10 is adapted to not operate the plotter mechanism 12 unless there is media properly disposed between the drive drum and the pinch roller. For this purpose, there is a sensor included among the sensors represented by block 16. For example, in a prior art pen plotter manufactured by the assignee of this application, a spring-biased spring was urged upwardly through a hole in a platen surface over which the media passes to be properly disposed between the drive drum and the pinch roller. When the media passed over the pin, the pin was forced down to provide a mechanical/electrical indication of detection of the media edge.
Also, for noise and energy conservation reasons, the plotter control logic 10 is adapted to turn on the motor servos for the vacuum fans represented by the fan 14 only when the media was in position and the pinch rollers were lowered onto the drive roller so that the plotter was ready to plot. In the same prior art plotter of the assignee of this application mentioned above, the pinch rollers were lifted up, a switch was activated with turned the motor servos off.
As can be appreciated, the foregoing prior art approach required to mechanical switches and accompanying switch activating mechanisms. Such apparatus adds to the manufacturing cost of a pen plotter and adds to its susceptibility to breakage as mechanical linkages and switches are more prone to breakage from metal fatigue and contact arcing and pitting, for example.
Wherefore, it is the object of the present invention to provide sensor input to the plotter control logic of a pen plotter which will allow the logic to sense the presence of a media leading edge in position and control the operation of the motor servos for the vacuum fans in a simple and reliable way.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.